Macaques infected with SIVDnef have been shown previously to produce lowered virus loads, remain healthy for more than 3 years, and resist challenge with virulent SIVmac251. However, SIVDnef infection is reported to be pathogenic to neonates. Compared to SIVDnef, SIVhyifn generated a reduced viral load and conferred greater resistance to challenge with a lethal dose of SIVmac251 in juvenile macaques. In this study we wanted to determine whether a live-attenuated SIV that expresses IFN-g (SIVhyifn was still pathogenic for newborn macaques. Three neonate macaques were inoculated orally with 1x105 TCID50 of SIVhyifn twice in a 24 hour period. Blood was collected weekly, and viral titers were measured in PBMCs and plasma. Virus was isolated from PBMCs from all animals by week one. The animal with the highest load reached a maximum titer of 100 TCID50/106 PBMCs on week 2; this was also the only timepoint when virus could be isolated from plasma. By week 6 no virus could be detected in any animal. Virus isolated from macaques was characterized by PCR amplification of the 3ULTR region, and no virus with the IFN-g insert was detected past week 1. The deletion of IFN-g insert was not manifested by increased virus replication; at 20 weeks postinoculation, all animals are still negative for virus isolation from PBMCs.